Dielectric Properties of Douglas-Fir Measured By Microwave Frequencies

A commercial microwave dielectrometer was used to measure the dielectric constant and loss tangent of Douglas-fir specimens at frequencies of 1, 3, and 8.53 GHz (1 GHz equals 10 cycles per second). Measurements were made at room temperature on groups of specimens ranging in moisture content from about 6 to over 30 percent. Values of the dielectric constant increased with increasing moisture content, with a slight concave upward trend at moisture levels below fiber saturation. A general decrease in dielectric constant with increasing frequency was observed. The loss tangent at 1 and 3 GHz increased sharply with increasing moisture content at low levels of moisture and appeared to be maximum at some moisture level near fiber saturation. The loss tangent at 8.53 GHz increased with increasing moisture content with a well-established concave upward trend. The loss tangent either remained constant or increased slightly with increasing frequency, depending on the moisture level and grain orientation.

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